グループ化 - Group-Object

前提

 gci | select name, extension | ft -auto

 Name  Extension
 ----  ---------
 1.txt .txt
 2.txt .txt
 3.txt .txt
 4.csv .csv
 5.csv .csv
 6.doc .doc

拡張子でグループ化

 gci | group-object extension | ft -auto

 Count Name Group
 ----- ---- -----
     3 .txt {1.txt, 2.txt, 3.txt}
     2 .csv {4.csv, 5.csv}
     1 .doc {6.doc}